This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Ribosomal processing requires a series of endo- and exonucleolytic steps for the production of mature ribosomes, of which most have been described. To ensure ribosome synthesis, 3'end formation of rRNA uses multiple nucleases acting in parallel;however, a similar parallel mechanism had not been described for 5'end maturation. Here, we identify Rrp17p as a previously unidentified 5'-3'exonuclease essential for ribosome biogenesis, functioning with Rat1p in a parallel processing pathway analogous to that of 3'end formation. Rrp17p is required for efficient exonuclease digestion of the mature 5'ends of 5.8S(S) and 25S rRNAs, contains a catalytic domain close to its N terminus, and is highly conserved among higher eukaryotes, being a member of a family of exonucleases. We show that Rrp17p binds late pre-60S ribosomes, accompanying them from the nucleolus to the nuclear periphery, and provide evidence for physical and functional links between late 60S subunit processing and export. A paper describing this work has been published (M. Oeffinger, D. Zenklusen, A. Ferguson, K.E. Wei, A. El Hage, D. Tollervey, B.T. Chait, R.H. Singer, M.P. Rout "Rrp17p Is a Eukaryotic Exonuclease Required for 50 End Processing of Pre-60S Ribosomal RNA" Molecular Cell, 36 (2009) 768-781).